It is known to provide a casting apparatus, generally consisting of a casting ladle from which a casting stream emerges from its lower end and a casting mold into which the casting stream is fed, with means for surrounding the liquid stream with a protective atmosphere adapted to prevent environmental oxygen from being solubilized in the melt or reacting therewith. It has been found that atmospheric oxygen has a tendency to react with components of the melt to produce scale or slag which detrimentally affects the quality of the casting produced.
The problem is especially pronounced when a so-called continuous casting is to be formed and the molten metal is fed substantially continuously to such a mold from which the solidifying metal emerges at a lower end or, at least, at a downstream end.
To avoid the detrimental effect of atmospheric oxygen on the casting stream, it has been proposed heretofore to direct along the casting stream between the casting ladle and the mold, an inert protective gas, i.e. a protective gas which does not react with the molten-metal stream but which serves to prevent access thereto by atmospheric oxygen.
For many reasons, nitrogen has been found to be the best protective gas for this purpose. For example, it is practically nonreactive and insoluble in the molten metal and, moreover, is relatively inexpensive, available in highly pure form and has a density and molecular weight such that it is effective to exclude the ambient atmosphere from contacting the stream.
Various techniques, including those described in the aforementioned application, have been used to ensure that a complete and uninterrupted blanket of the protective atmosphere will be interposed between the ambient atmosphere and the stream. For example, the casting stream can be led from the ladle to the mold through a tube or duct through which the inert gas is passed. This duct can be connected to the bottom of the casting ladle and, moreover, the protective atmosphere can, via gas distributors surrounding the stream, be directed along the latter.
This technique has, however, a significant disadvantage. Because the melt, i.e. the casting stream, contains dissolved oxygen, in spite of the fact that it is surrounded by the protective gas atmosphere, there is a significant reaction upon cooling of the melt between the entrained oxygen and any deoxidation agents included in the melt, thereby forming a slag which has the disadvantages mentioned above. In other words, the protective atmosphere, while preventing access of ambient oxygen to the casting stream, cannot counteract the effect of entrained oxygen which has previously solubilized in the melt.